What is an atom?

Much of the interesting science in modern electronics stems from the properties of quantum-size objects, at very small scales. So the most sensible place to start is at the bottom, with atoms.

There are many different kinds of matter, but most of the matter on Earth that humans interact with is made from atoms. What makes atoms non-intuitive for many people is their size, which is so small that we are able to get through day-to-day life never having to think about the atomic nature of matter. An atom like hydrogen is so small that you could line up ten million of them in a millimeter. In the same way that the universe is made of trillions of stars, in differing arrangements made up of differing materials, our bodies are made of trillions upon trillions of atoms.

A helium atom. The inset shows a detail of the nucleus, which has two protons and two neutrons. The entire atom is about an angstrom in size, which is less than a billionth of a meter.

Atoms were originally proposed as indivisible units of matter, hence the name which comes from a Greek word meaning indivisible or unable to be cut. However, what we now call atoms do have smaller objects inside them. Atoms are comprised of a central ball of protons and neutrons, called a nucleus, surrounded by a cloud of electrons. The nucleus is very heavy compared to the electron cloud, because protons and neutrons are approximately 2,000 times heavier than electrons. This means that in a collection of atoms, there are many dense nuclei, and the disperse electron clouds around them. Much of what we perceive as solid matter is actually made up of empty space, because of the low density of electron clouds. Nothing on Earth as dense as a nucleus is big enough to see by eye.

Within an atom, there can be different numbers of protons, neutrons, and electrons. What name we give to an atom is depends on the number of protons in its nucleus. The number of protons and the number of neutrons affect the size of the atomic nucleus. So if we compare a nucleus with one proton and no neutrons, which is a kind of hydrogen, with a nucleus that has eight protons and eight neutrons, which is a kind of oxygen, we find that the oxygen nucleus is physically larger than the hydrogen nucleus.

The number of electrons in an atom has a large effect on how the atom interacts with other atoms and with its surroundings. The numbers of protons and electrons impact each other as well, which we’ll get into when we go deeper into the quantum nature of atoms. But the numbers of protons, neutrons, and electrons give us many types of atoms, also called atomic species, which can be used as building blocks for matter.

But, from here we can begin to talk about what makes materials different, which largely depends on atomic properties and interactions. Even though many people don’t think about atoms when they go out into the world, much of our lives does stem from happenings in the world of atoms.